1. Field of the Invention
The present invention relates to a catalyst useful for purifying exhaust gases from vehicles, and more particularly, to a catalyst useful for purifying exhaust gases from a diesel engine. The catalyst has increased efficiency for purifying both nitrogen oxide and soot particles (particulates) from the exhaust emissions of the diesel engine.
2. Description of the Related Art
There is an increasing world-wide interest in preserving the environment, along with other environmental concerns. One type of pollution, air pollution, as opposed to water and soil pollution, is caused primarily by combusters, (e.g., combustion engines), and is seriously affected by the stricture of the exhaust system of the combuster, the operating principle thereof, and weather conditions. A typical combuster which causes significant air pollution, is motor vehicle combustion engines.
The significance of the air pollution caused by vehicles is that vehicles emit pollutants wherever they go, and the use of vehicles sharply increases with improvement in living conditions. For this reason, various regulatory agencies have imposed restrictions on the exhaust emissions from vehicles. As a result of efforts made to satisfy and comply with these restrictions, development and use of a three-way catalyst and a lean burnt catalyst has achieved almost complete purification of carbon hydroxide, carbon monoxide and nitrogen oxide in the exhaust emissions from gasoline engines.
The problems associated with diesel engines, however, are different from the problems associated with gasoline engines. In addition, the use of diesel engines has greatly increased due to the high combustion efficiency of diesel and its low cost, when compared to gasoline. Due to the combustion principle of diesel engines which burn diesel under high-pressure, and in an oxygen-rich atmosphere, diesel engines emit solid and liquid composite pollutants such as soot particles (particulates), nitrogen oxides, soluble organic substances, sulfides, etc. In particular, particulates containing a carcinogenic substance such as a multinucleate aromatic substance are considered to be the most harmful exhaust emissions, and are emitted in the form of an undesirable visible smog. For this reason, there has been significant research into development of an exhaust gas purification system for diesel engines, which has been focused primarily on the development of a catalyst that is useful for removing such particulates.
Nitrogen oxides, another major component of the exhaust emissions from diesel engines, are considered to be the main portion of air pollution that causes acid rain, as well as the formation of ozone and smog by reaction with hydrocarbons. Nitrogen oxides can be eliminated by a reduction reaction. However, the presence of excessive oxygen relative to the oxygen equivalent required for oxidizing combustible carbonic compounds, such as un-burned hydrocarbons or carbon monoxide, hinders the natural elimination of nitrogen oxides via reduction. In addition, it is difficult to purify nitrogen oxides under a sulfur oxide (SO.sub.x) atmosphere, and particularly, under a sulfur dioxide (SO.sub.2) atmosphere.
Existing exhaust gas purification techniques for diesel engines typically are classified into two classes: (i) for burning particulate materials like soot and the like using a trap; and (ii) for burning soluble organic substances with a flow-through type catalyst produced by coating an open-cell honeycombed carrier with a catalyst. In particular, the trap for burning particulate materials is usually used exclusively for removing carbon from the particulates. For this reason, it cannot be used for removing the nitrogen oxides. In contrast, the flow-through type catalyst for burning organic substances is usually used to oxidize the soluble organic substances, in addition to the particulates, hydrocarbons and carbon monoxide. This oxidation process serves to eliminate these soluble organic substances, particulates, hydrocarbons, and carbon monoxide by using a metal catalyst in an oxide carrier, in a similar fashion to catalysts for gasoline engines. In addition, it is known that the use of the flow-through type catalyst can reduce nitrogen oxides to some extent.
However, since the existing flow-through type catalyst mainly burns the soluble organic substances, the reduction activity of the catalyst with respect to nitrogen oxides is merely at 20-30 percent of its oxidation activity with respect to particulates. This is believed to be because sulfur in diesel produces excessive sulfur dioxide, oxygen, and water in the exhaust emissions, which results in reduced activity and durability of the catalyst.
Catalysts useful for purifying exhaust gases from vehicles usually are comprised of a carrier and a main catalyst. Typical examples of the carrier, which has its inherent activity and is a decisive factor in determining the characteristics of the purification catalyst, include alumina, titanium dioxide, zirconium dioxide, silicon dioxide, and the like. Alumina, when used for diesel engines, however, has the disadvantage in that it adsorbs sulfur dioxide at low temperatures and emits sulfur trioxide at high temperatures via oxidation. This oxidation increases particulates in the exhaust emissions and reduces the activity and durability of the catalyst.
Titanium dioxide and zirconium dioxide, which are used alone or in a mixture, adsorb only a small amount of sulfur dioxide and produce only a small amount of sulfate, but exhibit a sharp reduction in their specific surface area at high temperatures.
These oxides therefore cannot sufficiently exert their functions as a carrier. In addition, titanium dioxide and zirconium dioxide lower the activity of precious metals and transition metals, and in turn deteriorate the catalyst. Silicon dioxide has a strong resistance against the toxicity of both sulfur dioxide and water, but due to its low activity, a large amount of catalyst needs to be impregnated therewith.
Catalysts useful for purifying exhaust gases from vehicles typically are comprised of precious metals. Platinum (Pt) and palladium (Pd), which are typical precious metals used in a three way catalyst for gasoline engines, are known as effective catalysts due to their considerably high purification activity with respect to nitrogen oxides, in addition to hydrocarbons and carbon monoxide. Accordingly, Pt and Pd have also been used widely for purification of the exhaust gas from diesel engines.
Although Pt has an advantage of exhibiting good purification activity for nitrogen oxide in diesel engines operating under an oxygen-rich atmosphere, it has a disadvantage in that it facilitates oxidation of sulfur dioxide at a temperature of 300.degree. C. or more. Pt also serves as crystal nuclei for particulates, thereby increasing the amount of particulates in the exhaust. Adding vanadium oxides has been proposed to account for this problem, due to their ability to suppress the oxidizing power of sulfur dioxide. However, vanadium oxides reduce the oxidation activity for pollutants including particulates, hydrocarbons, and carbon monoxide, along with the oxidizing power of sulfur dioxide, thereby lowering the durability of the catalyst.
While Pd has an advantage in that it facilitates the oxidation activity for sulfur dioxide at fairly high temperatures, for example, at at least 450.degree. C., it has a low oxidation activity for pollutants at low temperatures and a reduced durability at low temperatures.
In terms of cost and limited reserves of precious metals, there is a need for new substitutes for precious metals. However, since a main catalyst component capable of satisfactorily substituting for a precious metal has not yet been found, the amount of the precious metal used has been reduced with the aid of co-catalysts such as transition metals, rare earth metals, and oxides of these metals. However, these co-catalysts have a low initial activity, and are adversely affected by sulfur dioxide and water, which results in reduced durability.